1. Field of the Invention
The present invention relates to a surface wave circuit device, and more particularly to a structure of an electro-acoustic/acoustic-electrical transducer/reflector used in circuit devices operated in the VHF or UHF frequency band, such as resonators, filters and oscillators, which has conductive metal strips, slits or grooves on a surface of a substrate of piezoelectric material or non-piezoelectric material which can propagate acoustic wave.
2. Description of the Prior Art
In the circuits used in the VHF or UHF frequency band, circuit devices utilizing surface acoustic waves, such as resonators and filters, have been developed to take place of conventional circuit devices utilizing lumped circuit elements such as capacitors, resistors and inductors, and they have been used in communication equipment and television receivers. Although such surface wave circuit devices have many advantages such as small size, stability in operational characteristics, uniformity of device characteristics and economy of cost, they still have problems to be resolved. Specific problems to be resolved are to reduce losses and to attain a desired frequency characteristic.
Primary elements to determine the frequency characteristic of the surface wave circuit device are an acoustic-electrical transducer and a reflector.
The transducer and the reflector include a number of parallelly arranged elongated metal stripes, slits or grooves on a piezoelectric or non-piezoelectric substrate. For the transducer, metal stripes are interleaved and positive and negative voltages are applied thereto or positive and negative voltages are taken therefrom.
In these transducers or reflectors, the following two major approaches have been used to attain desired frequency response. (For the sake of simplicity, a transducer having interdigital electrodes is discussed below.)
FIG. 1 shows a transducer which is known as an apodized transducer. As shown on the left of FIG. 1, two comb-shaped electrodes 1 and 2 are arranged such that stripes (electrode fingers) of one electrode are inserted between stripes of the other electrode with a crossing region of the stripes presenting a function f(x) which approximates an impulse response of the transducer. That is, weighting is effected by crossing region modulation of the electrode fingers of the interdigital electrodes. (A variable x represents a position in the direction of propagation of the surface wave). A transducer shown in FIG. 2 is known as a withdrawal transducer. As shown on the left of FIG. 2, density of electrode pairs presents the function f(x) which approximates the impulse response of the transducer.
The transducer shown in FIG. 1 induces a beam-shaped surface wave close to a Gaussian function as shown by 3 in FIG. 1 which has a larger amplitude at the center of the transducer and a smaller amplitude at upper and lower ends. When the input transducer is an apodized transducer and the output transducer is a conventional unapodized transducer, a lateral length S of the output transducer must be larger than a maximum crossing length J of the electrode fingers of the input transducer in order to enable the receiving transducer to receive all of the transmitted waves, while taking the spread of the acoustic wave due to diffraction into consideration. However, it is known that when the beamshaped acoustic wave having non-uniform amplitudes is received by an output transducer of large lateral length S, the receiving efficiency (i.e. transfer efficiency in the receiving transducer to transduce acoustic energy to electric energy) is much lower than that when it receives a uniform plane wave. Accordingly, it is difficult to attain a low-loss surface acoustic filter.
In the transducer shown in FIG. 2, the amplitude of the acoustic wave induced by the transducer is a uniform plane wave so that the diffraction of the acoustic wave is small and the receiving efficiency is high. However, since a desired frequency response is to be attained by the withdrawal weighting of the electrode fingers, a large number of electrode pairs of the transducer are required so that the application thereof is usually limited to a narrow band filter. It is inferior to the apodized transducer when a complicated frequency response is desired.